The widespread use of mobile electronic devices such as smartphones and tablets has created a need in the art for compact and efficient switching power converters for recharging the batteries of these devices. A flyback switching power converter is typically provided as the charger for a mobile device as its transformer provides safe isolation from AC household current. The power output of the converter needs to be controlled, however, to avoid or prevent damage to devices connected to the converter or to the converter itself.
Conventional flyback converters typically provide output regulation using a controller, which controls the ON and OFF state of a power switch. The controller regulates an output voltage (or current) by cycling the power switch responsive to a number of signals to control the operation of the power switch. One of those signals may be derived from sensing the current through the power switch when the power switch is in the ON state by in turn sensing the voltage across a sense resistor in series with the power switch such that the power switch current flows through the sense resistor. The primary winding current (which is the same as the power switch current) is then proportional to the voltage drop of the sense resistor and may be represented by a voltage, V_Isense, as shown in FIG. 1 for a cycle of a power switch (S1).
When the controller places an NMOS power switch (S1) transistor in the ON state at time t0, the drain voltage for the power switch (V_DRAIN_S1) goes low to ground. The primary winding current flows through the power switch and the sense resistor, developing the sense resistor voltage (V_Isense) signal. The controller senses the peak primary winding current by sensing the V_Isense. In normal operation, the controller employs a peak current threshold (V_IPeak) to determine the proper moment to place the power switch back into the OFF state by attempting to determine when the current through power switch reaches the peak current threshold. But if the sense resistor is shorted (or soft-shorted) due to a fault on the sense resistor, V_Isense will never reach the desired peak current threshold. Instead, the voltage across the sense resistor (V_Isense) would barely increase above ground as shown in FIG. 1 in the case of a soft-short circuit (very low impedance) for the sense resistor. In the case of a hard short circuit, this voltage would stay grounded. Should the controller continue to maintain the power switch on, the primary winding current will continue to increase such that the power switch (or other components) may be harmed by the excess current, resulting in a permanent failure of the power switch. Moreover, such a large primary winding current will tend to drive the output voltage for the power converter out of regulation. It is thus conventional for the controller to employ a maximum on-time timer that times a maximum on-time period starting when the power switch is switched on at time t0. In FIG. 1, the maximum on-time period extends from time t0 to a time t1. Should the maximum on-time period expire without the peak current threshold being reached, the power converter detects a sense resistor short fault condition and enters, for example, a reset operation.
Although such a method can detect short circuit conditions at the sense resistor, it can also indicate false positives (e.g., indicate a short circuit that does not actually exist) under a number of conditions, because the rate of rise of the S1 switch current is directly affected by the rectified input voltage to the converter. For example, a brief drop on the AC mains voltage from which the rectified input voltage is obtained may cause the V_Isense voltage to ramp up more slowly such that it does not cross the peak current threshold prior to expiration of the maximum on-time period.
Accordingly, there is a need in the art for improved detection of sense resistor short conditions in switching power converters.